Various processes have heretofore been proposed for mechanical plating techniques which employ blasting to form a protective coating on the surface of metallic materials, especially iron.
For instance, British Pat. No. 1,041,620 discloses a process for forming a corrosion-resistant coating by blasting a mixture of grit and particles of a coating metal onto the surface to be treated. Zinc powder is given as an example of the coating metal particles, and this patent teaches that the zinc powder should preferably be of high quality containing not more than 0.2% by weight of lead, arsenic, etc, and that steel shot which is harder than the coating metal particles, especially steel shot having a particle diameter of 0.4-0.8 mm, is preferred as grit. However, although a zinc coating film is formed by the process of this British Patent, the coating amount of the formed zinc film is limited and the corrosion resistance thereof is also limited as illustrated hereinafter by Comparative Examples A and B. It is thought that this is because the zinc particles are soft and smooth and therefore they are easily flattened between the shot material and the surface being treated. This flattening absorbs the energy of projection, and increases the collision contact area between the surface being treated and the zinc particles, making exposure of active surface difficult and thus decreasing adhesion.
Japanese Laid-Open Patent Publication No. 12405/72 discloses a mechanical plating material comprising shot material on the surface of which a coating metal (zinc) powder is bonded with an organic binder. This material differs from that of said British Patent in that an organic binder is used. But the film thus formed is limited in coating amount and corrosion resistance because of the smooth surface and the low hardness of zinc, since high purity zinc is used.
Thus the processes of said British Pat. No. 1,041,620 and Japanese Laid-Open Patent Publication No. 12405/72 practically did not achieve commercial success.
Japanese Laid-Open Patent Publication No. 21773/81 and Japanese Patent Publication No. 9312/84, the applicants of which are the same as the assignee of the present invention, disclose shot materials which comprise iron cores around which iron-zinc alloy (intermetallic compound) crust is integrally formed with the cores. When these blast materials are used, the iron-zinc alloy, which is very hard and brittle, is broken by brittle fracture by the percussion energy of the heavy iron cores and the broken particles hit the surface being treated with a small collision contact area, and thus a tightly-bonded film (iron-zinc alloy film) with very good corrosion-resistance is formed with a high coating weight. Therefore, these materials are now being spotlighted as promising commercial mechanical plating materials. However, a problem with these materials is how to carry out the mechanical plating continuously with the materials without time course change, replenishing the consumed iron-zinc layer. With respect to this problem, Japanese Laid-Open Patent Publication No. 9312/84 suggests that the initially-used blast material is supplied as the operation continues. In this case, the particles which have been abraded during the operation inevitably remain in the blast material system.
Japanese Laid-Open Patent Publication No. 93801/81 discloses a zinc alloy powder for mechanical plating, which comprises zinc to which small amounts of various metals are alloyed. This coating metal powder does not contain iron as an alloying element.
Japanese Patent Publication No. 25032/84 discloses a mechanical plating method for forming corrosion-resistant film by adjusting the particle size of the shot material and that of the coating material. This publication does not teach use of iron-zinc alloy.
The present invention provides a novel and useful blast material for mechanical plating and a continuous mechanical plating process which are different from the conventional mechanical plating techniques.